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At least 145 records · Page 8
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NASA NTRS1989

IUE observations of the M dwarfs CM Draconis and Rossiter 137 B - Magnetic activity at saturated levels

IUE observations of two active M dwarfs with known rotation rate or age and presumed to be almost totally convective are presented. The first of these stars, CM Draconis (Gl 630.1), is an old Population II binary with its components in tidally induced rapid rotation (P = 1.27 d, dM 4 + dM 4). The other one, Rossiter 137 B, forms with HD 36705 (AB Dor) a visual pair of young active stars. The activity of CM Dra is due to the forced rotation in a close binary, while Rst 137 B is assumed to rotate fast enough to generate its magnetica activity. These results are compared with those for M dwarfs, particularly AU Mic and YZ CMi, that have known rotational periods and measured ultraviolet emission line fluxes. The chromospheric-coronal saturation levels of cool dwarfs between 0.3 less than B-V less than 1.6 is determined. It is found that saturated F stars have stronger chromospheres than saturated M stars, but the opposite is true for the corona. Results of these observations seem to favor a rotation-dependent distributed dynamo generating magnetic flux in totally convective stars.

Vilhu, O.↗
NASA NTRS1994

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis stars. 18: Coordinated VLA, ROSAT, and IUE observations of RS CVn binaries

As part of a coordinated program of multi-wavelength observations of RS CVn close binary systems, we observed 15 systems with the Very Large Array (VLA) and 10 systems with IUE, simultaneously or nearly simultaneously with the ROSAT All Sky Survey observations of these stars. Of the 22 systems observed with ROSAT, three were observed both by IUE and the VLA. Radio observations were made at 3.6, 6 and 20 cm. Of the 15 observed RS CVn systems, we detected 11 with greater than 4 sigma confidence at one or more wavelengths. The IUE observations were made within the RIASS (ROSAT-IUE All Sky Survey) program. We present the results of the VLA observations, along with the corresponding subsets of the ROSAT Position Sensitive Proportional Counter (PSPC) X-ray and Wide Field Camera (WFC) XUV survey, and RIASS IUE observations. We obtained an extended VLA/IUE/ROSAT simultaneous coverage of one system, TY Pyx, covering more than one orbital period. These observations reveal that the quiescent radio flux of TY Pyx is relatively constant over time scales of up to 7 hours, but that it did change by a factor of 3 over 24 hours, probably due to a flare on 1990 Nov. 12. The UV, XUV and X-ray fluxes do not show large day-to-day or phase-related variability. The observation of the decay phase of a radio flare on EI Eri, with no accompanying X-ray or XUV flare, suggests that the lack of a strong correlation between X-ray and radio flares previously noted for dMe flare stars holds for RS CVn systems as well. We suggest that the radio flare may have been due to a coherent emission process such as electron cyclotron emission. The simultaneous measurements presented here provide a unique test of the general correlation between radio and soft X-ray luminosities, L(sub radio) approximately L(sub x exp m) (Drake et al. 1989) with a power-law slope close to unity, which was previously derived using data obtained years apart. Our derived slopes are consistent with and thus support the general correlations between coronal and chromospheric/transition region emissions previously derived from nonsimultaneous measurements of a much larger sample of these variable sources. However, the importance of simultaneous measurements for accurate energy balance calculations is stressed.

Fox, David C.↗
NASA NTRS1986

Triple interactions involving close binaries in globular clusters

The interaction of a circularized tidal binary system with a third star is studied statistically through the results of about 70,000 numerical scattering experiments. The stars involved are taken to be identical polytropes, with radii equal to one-third of the initial binary separation. The effects of nonzero stellar radii are included by calculating an approximation to the energy dissipated by tidal interactions and estimating the energy lost through physical collisions. These are then used to modify the interaction cross sections obtained in the point-mass approximation. It is found that, for a wide range of assumptions about the details of the triple interactions and the stars' structure, the net rate at which tidal binaries heat the stellar system is substantially reduced (by as much as a factor of 20) below that obtained when the binaries are treated simply as point masses. The most likely outcome of a triple encounter is the coalescence of two, or all three, of the stars involved.

Mcmillan, S. L. W.↗
NASA NTRS1989

A compact planetary nebula around the hot white dwarf EGB 6/PG 0950 + 139

The remarkable central star (0950 + 139), a very hot DA/DAO white dwarf, of the planetary nebula EGB 6 is described. Follow-up observations relevant to the analyses of both the nebula and the stellar photosphere are presented. Three kinds of scenarios are discussed to account for the existence of this peculiar nebula, but none appears very promising. The first consideration is that the nebula was ejected from the white dwarf as a discret event. This hypothesis is heavily constrained by the nebular size, density, and expansion rate; by the low luminosity and radius of the star; and by the absence of evidence for variation in density-sensitive forbidden lines from 1978 to 1987. No plausible mechanism can cause the observed amount of mass to be lost directly from a white dwarf in a steady or sporadic wind, at outflow velocities orders of magnitude below the escape velocity. Final consideration is given to the possibility that the gas is lost from a close companion star, but there is no evidence that this is a close binary system.

Liebert, James↗
NASA NTRS1994

Astronomical observations of phenomena in disks

Astronomical constraints on disks around young stellar objects are reviewed in this abstract. Since disks around the youngest stars cannot generally be resolved spatially, most results are based on analyses of the infrared and radio emission from circumstellar dust and gas. At least half of all young stars seem to have substantial circumstellar disks at an age of approximately 10(exp 6) yr. The young objects without disks tend to be binary stellar systems, though not exclusively, and some close binaries may have outer disks that are not tidally disrupted. Probably most, if not all, stars formed originally with circumstellar disks, but evolutionary timescales may differ dramatically. All the results, disk disappearance, disk mass around young stars, and long-wavelength dust opacities, are subject to the additional uncertainty of the effects of infalling dusty envelopes, which produce the disks in the first place.

Hartmann, L.↗
NASA NTRS1991

Observational tests of convective core overshooting in stars of intermediate to high mass in the Galaxy

This study presents the results of 14 tests for the presence of convective overshooting in large convecting stellar cores for stars with masses of 4-17 solar masses which are members of detached close binary systems and of open clusters in the Galaxy. A large body of theoretical and observational data is scrutinized and subjected to averaging in order to minimize accidental and systematic errors. A conservative upper limit of d/HP less than 0.4 is found from at least four tests, as well as a tighter upper limit of d/HP less than 0.2 from one good test that is subject to only mild restrictions and is based on the maximum observed effective temperature of evolved blue supergiants. It is concluded that any current uncertainty about the distance scale for these stars is unimportant in conducting the present tests for convective core overshooting. The correct effective temperature scale for the B0.5-B2 stars is almost certainly close to one of the proposed hot scales.

Stothers, Richard B.↗
NASA NTRS1996

Planetesimals Around V536 AQL

The pre-main sequence star V536 Aql, classified as a K7 classical T Tauri star by Cohen & Kuhi has been resolved, by high angular resolution near-infrared speckle observations, as a close binary (0.52 sec separation at 17 deg) surrounded by extended structures. These structures seen, both in the July 1993 and April 1994 observations, are not at the same position in the reconstructed image and do not present exactly the same shape at both times. Although it is unlikely that the presence of these structures is due to seeing calibration problems, the exact shape might be affected by it. We cannot presently make a final interpretation of the observations but can formulate different possibilities: the 'circumstellar' material seen in our images can be an independent cloud, or simply gas or dust, in front of the system on our line of sight, but close enough to still be illuminated by the binary; this material may be 'by chance' there or be a remnant of the material in which the star was born. We propose a model to explain the position variation of the extended structures. New observations are under reduction and should help to determine the exact value of this motion. At their distance from the star, motions of the extended matter would correspond to velocities much higher than the Kepler velocities. To explain this, we have developed a model based on a 'torch-light' effect. The possibility that the observed elongated structures belong to a circumstellar or circumbinary disk are discussed.

Ageorges, N.↗
NASA NTRS1989

Masses and the evolution of Wolf-Rayet binaries

Recent polarimetric observations have provided inclinations of double-line spectroscopic binary systems containing Wolf-Rayet stars with O star companions. Therefore, reasonably accurate masses of ten Wolf-Rayet stars in binaries are now known. These data are compared to evolutionary models of massive close binaries based on the scenario that O + O systems evolve into WR + O systems. Good agreement is found between the observations and nonconservative binary models with classical cores for the observed range of contemporary masses. The models with large convective-core overshooting fail to describe systems with Wolf-Rayet stars having observed masses larger than about 10 solar masses.

Schulte-Ladbeck, Regina E.↗
NASA NTRS2010

The Spatially-resolved Interacting Winds of Eta Carinae: Implications on the Orbit Orientation

Medium-dispersion long slit spectra, recorded by HST/STIS (R=8000, Theta=0.l"), resolve the extended wind-wind interaction region of the massive binary, Eta Carinae. During the high state, extending for about five years of the 5.54-year binary period, lines of [N II], [Fe III], [S III], [Ar III] and [Ne III] extend outwards to 0.4" with a velocity range of -500 to +200 km/s. By comparison, lines of [Fe II] and [Ni II] extend to 0.7" with a velocity range of -500 to +500 km/s. During the high state, driven by the lesser wind of Eta Car B and photo-ionized by the FUV of Eta Car B, the high excitation lines originate in or near the outer ballistic portions of the wind-wind interaction region. The lower excitation lines ([Fe II] and [Ni II D originate from the boundary regions of the dominating wind of Eta Car A. As the binary system has an eccentricity exceeding 0.9, the two stars approach quite close across the periastron, estimated to be within 1 to 2 AU. As a result, Eta Car B moves into the primary wind structure, cutting off the FUV supporting the ionization of the high state lines. Forbidden emission lines of the doubly-ionized species disappear, He II 4686 drops along with the collapse of the X-ray flux. This behavior is understood through the 3-D models of A. Okazaki and of E. R. Parkin and Pittard. Discussion will address the orbit orientation relative to the geometry of the Homunculus, ejected by Eta Carinae in the 1840s.

Gull, Theodore R.↗
NASA NTRS1993

The evolution of the lithium abundances of solar-type stars. IV - Praesepe

Echelle observations are presented of lithium in 63 F and G dwarfs of the Praesepe cluster. For stars earlier than about G0V, Praesepe follows the same trends seen in the Hyades, which has approximately the same age and composition. Stars in Praesepe later than about G5V have more Li than their Hyades counterparts, possibly because Praesepe is slightly younger than the Hyades or has slightly lower metallicity. Significant differences in the abundance of Li are seen among stars of the same color, and, as in the Hyades, there is a tendency for the deviant stars to be binaries to the extent that duplicity in Praesepe is known. There are also stars with much less Li than most cluster members yet which appear to be true members of Praesepe. The close binary KW 181 has a normal Li abundance, despite the fact that similar close binaries in the Hyades are Li rich.

Soderblom, David R.↗
NASA NTRS1988

Radio emission from RS CVn binaries. III - A VLA survey of 103 systems

The paper presents a 5 GHz radio survey of 103 close late-type stellar binaries. Fifty-three of the 103 candidate stars were detected above an average detection threshold of 0.4 mJy. The median observed radio luminosity is compared with total bolometric luminosity, rotational period, color index (B-V), and Rossby number.

Morris, D. H.↗
NASA NTRS2012

Marginally Stable Nuclear Burning

Thermonuclear X-ray bursts result from unstable nuclear burning of the material accreted on neutron stars in some low mass X-ray binaries (LMXBs). Theory predicts that close to the boundary of stability oscillatory burning can occur. This marginally stable regime has so far been identified in only a small number of sources. We present Rossi X-ray Timing Explorer (RXTE) observations of the bursting, high- inclination LMXB 4U 1323-619 that reveal for the first time in this source the signature of marginally stable burning. The source was observed during two successive RXTE orbits for approximately 5 ksec beginning at 10:14:01 UTC on March 28, 2011. Significant mHz quasi- periodic oscillations (QPO) at a frequency of 8.1 mHz are detected for approximately 1600 s from the beginning of the observation until the occurrence of a thermonuclear X-ray burst at 10:42:22 UTC. The mHz oscillations are not detected following the X-ray burst. The average fractional rms amplitude of the mHz QPOs is 6.4% (3 - 20 keV), and the amplitude increases to about 8% below 10 keV.This phenomenology is strikingly similar to that seen in the LMXB 4U 1636-53. Indeed, the frequency of the mHz QPOs in 4U 1323-619 prior to the X-ray burst is very similar to the transition frequency between mHz QPO and bursts found in 4U 1636-53 by Altamirano et al. (2008). These results strongly suggest that the observed QPOs in 4U 1323-619 are, like those in 4U 1636-53, due to marginally stable nuclear burning. We also explore the dependence of the energy spectrum on the oscillation phase, and we place the present observations within the context of the spectral evolution of the accretion-powered flux from the source.

Strohmayer, Tod E.↗
NASA NTRS1984

Egress spectra of the new eclipsing supergiant 22 Vulpeculae

The G-type primary in the single-lined spectroscopic binary 22 Vul (HD 192713, Gp3 Ib-II, period 249 days) was found by IUE to eclipse its recently identified B9 main sequence companion. The system thus joins Zeta Aurigae systems in which a hot dwarf provides a probe of extended atmospheres by going behind a cool supergiant. The IUE data were obtained at scattered dates following two conjunctions. Outside of eclipse, a wind from the system with terminal velocity of 300 k/sec is conspicuous in the Mg II lines. Enhanced Fe II absorption seen well away from geometrical eclipse implies an extensive envelope around the G star. There is a close correspondence between the 22 Vul spectra and those of interacting binaries, suggesting common properties due to relatively dense, warm plasmas in these systems.

Ake, T. B., III↗
NASA NTRS1982

The structure and energy balance of cool star atmospheres

The atmospheric structure and energy balance phenomena associated with magnetic fields in the Sun are reviewed and it is shown that similar phenomena occur in cool stars. The evidence for the weakening or disappearance of transition regions and coronae is discussed together with the appearance of extended cool chromospheres with large mass loss, near V-R = 0.80 in the H-R diagram. Like the solar atmosphere, these atmospheres are not homogeneous and there is considerable evidence for plage regions with bright TR emission lines that overlie dark (presumably magnetic) star spots. The IUE observations are providing important information on the energy balance in these atmospheres that should guide theoretical calculations of the nonradiative heating rate. Recent high dispersion spectra are providing unique information concerning which components of close binary systems are the dominant contributors to the observed emission. A recent unanticipated discovery is that the transition lines are redshifted (an antiwind) in DRa (G2 Ib) and perhaps other stars. Finally, the G and K giants and supergiants are classified into three groups depending on whether their atmospheres are dominated by closed magnetic flux tubes, open field geometries, or a predominately open geometry with a few closed flux tubes embedded.

Linsky, J. L.↗
NASA NTRS1981

Accretion disk coronae

Recent observations of partial X-ray eclipses from 4U1822-37 have shown that the central X-ray source in this system is diffused by a large Compton-thick accretion disk corona (ADC). Another binary, 4U2129-47, also displays a partial eclipse and contains an ADC. The possible origin of an ADC is discussed and a simple hydrostatic evaporated ADC model is developed which, when applied to 4U1822-37, 4U2129+47 and Cyg X-3, can explain their temporal and spectral properties. The quasi-sinusoidal modulation of all three sources can be reconciled with the partial occultation of the ADC by a bulge at the edge of the accretion disk which is caused by the inflowing material. The height of this bulge is an order of magnitude larger than the hydrostatic disk height and is the result of turbulence in the outer region of the disk. The spectral properties of all three sources can be understood in terms of Compton scattering of the original source spectrum by the ADC. Spectral variations with epoch in Cyg X-3 are probably caused by changes in the optical depth of the corona. A consequence of our model is that any accreting neutron star X-ray source in a semi-detached binary system which is close to its Eddington limit most likely contains an optically thick ADC.

White, N. E.↗
NASA NTRS1982

Hot wind from Gamma/2/ Velorum observed in the ultraviolet carbon lines

The close binary system Gamma(2) Vel, consisting of a WC8 and an O9 I star, has been observed in the ultraviolet at various phases. Preliminary analysis of selected data shows that the temperature of the stellar wind from this binary increases outward. This suggests that an additional source of energy other than radiation pressure may exist for the wind. The mass flow occurs in all directions from the binary. However, there is evidence of an increase in the mass flow through the third Lagrangian point, although from the extant data it is not possible to decide whether or not a similar increase in the mass outflow occurs through the second Lagrangian point. There is also evidence of mass surge, which probably is independent of orbital phase.

Kondo, Y.↗